1. Technical Field
The invention relates to a polarization conversion device which rotates a polarization direction of linearly polarized light by 90°, a polarized illumination optical device which converts non-polarized illumination light into illumination light of linearly polarized light having a fixed polarization direction by the polarization conversion device, and an liquid crystal projector which uses the polarized illumination optical device in an illumination optical system.
2. Description of the Related Art
Various kinds of liquid crystal projectors that illuminate an image, which is displayed on a liquid crystal display panel, with light from a light source lamp for projection onto a screen are commercially available. As is known well, a liquid crystal display panel includes a liquid crystal layer having a predetermined thickness, in which liquid crystal molecules are sealed, and a polarizer and an analyzer which are disposed on an incidence surface side and an emission surface side of the liquid crystal layer, respectively. The polarizer and the analyzer are disposed so that polarization directions of them are perpendicular or parallel to each other. The polarizer and the analyzer control passage of linearly polarized light, which is incident on the liquid crystal layer, according to an orientation state of liquid crystal molecules, and adjust a light amount of linearly polarized light which is emitted through the analyzer.
On the other hand, generally, a polarization conversion device which converts non-polarized light from a light source into linearly polarized light having the same polarization direction as the polarizer of the liquid crystal display panel is used in the illumination optical system of the liquid crystal projector. As described in JP 2008-129190 A, a prism array obtained by combining a polarized beam splitter and a ½ wavelength plate is often used as such a polarization conversion device. The polarized beam splitter has a polarization split surface which transmits one of two kinds of linearly polarized light beams whose polarization directions are perpendicular to each other, and reflects the other one. The polarized beam splitter rotates the polarization direction of either one of the two kinds of linearly polarized light, which are separated on the polarization split surface, by 90° using the ½ wavelength plate and combines it with the other linearly polarized light, thereby obtaining linearly polarized light whose polarization directions are aligned.
Most of known ½ wavelength plates are formed of a film sheet using an organic material. Accordingly, if the ½ wavelength plates are used near the light source for a long time, discoloration easily occurs and the polarization conversion efficiency is deteriorated. In order to improve the thermal resistance, it has been proposed to use a birefringent crystalline substance, such as crystal, as the ½ wavelength plate. However, not only the crystal itself is expensive, but also the manufacturing cost is high because the crystal should be processed while controlling the optical axis of the crystal precisely. From this point of view, in the polarization conversion devices described in JP 2008-129190 A and JP 2006-64871 A, a ½ wavelength plate formed of a dielectric multilayer is used. In this case, there are advantages in that not only the thermal resistance is greatly improved but also the manufacturing cost is suppressed to be low.
The ½ wavelength plate used in the polarization conversion devices described in JP 2008-129190 A and JP 2006-64871 A is formed of the dielectric multilayer which is manufactured by the oblique deposition. A retardation film formed of a dielectric multilayer which is manufactured by the oblique deposition may be put into practical use up to a ¼ wavelength plate. However, if the film thickness is increased up to a level at which the retardation film can be used as a ½ wavelength plate, the retardation film becomes clouded. As a result, the transmittance would be reduced. Particularly, a reduction in transmittance at the short wavelength side is noticeable, which would adversely affect the color balance. Also, the dielectric multilayer which is manufactured by the oblique deposition tends to absorb the moisture. As a result, the optical property of the dielectric multilayer would largely change as the dielectric multilayer absorbs the moisture.